1. Field of the Invention
The present invention relates to a cipher communication in which a communication between two terminals with a connection is realized by using a shared key cipher algorithm.
2. Description of the Background Art
The cipher communication system using a shared key cipher algorithm can be effectively utilized in realizing the cipher communication within a specific group of members such as business offices which are distributed over a wide range of regions as in a ease of the LAN (Local Area Network).
In the conventional cipher communication system, a plurality of terminals are connected with a key distribution center through a network, and a connection between any two terminals is established and the cipher communication between them is realized by obtaining a session key generated by the key distribution center. Here, in general, after the connection with a communication target terminal is established, an application program of one communication side makes an access to the key distribution center to acquire the session key to be used in the cipher communication, and this session key is transmitted to the communication target terminal such that this key is commonly shared and the cipher communication is realized by enciphering and deciphering the communication data by using this commonly shared key thereafter.
For example, a conventional cipher communication system has an exemplary configuration as shown in FIG. 1, in which a plurality of clients 3.sub.1 to 3.sub.3 and a server 5 are connected with a key distribution center 1 through a network 2 such that the client 3 or the server 5 can obtain the session key generated by the key distribution center 1.
In this conventional cipher communication system of FIG. 1, in a case of requesting the session key to the key distribution center 1 from the client 3 by using its application program, the cipher communication is realized by the procedure as shown in FIG. 2A, whereas in a case of requesting the session key to the key distribution center 1 from the server 5 by using its application program, the cipher communication is realized by the procedure as shown in FIG. 2B. Namely, after the session is established between the client 3 and the server 5, the client 3 or the server 5 obtains the session key to be used in communication by making an access from the application program to the key distribution center 1, and the obtained session key is shared between the client 3 and the server 5, such that the cipher communication is realized by enciphering and deciphering the communication packets by using this session key thereafter.
However, in such a conventional cipher communication scheme, before the cipher communication begins, there is a need for the application program executed on the terminal to establish a connection with the key distribution center, acquire the session key from the key distribution center, and transmit the same session key to the application program on the communication target terminal.
On the other hand, an enormous number of communication programs for carrying out communications among terminals through a connection oriented network are presently available, and in order to modify these communication programs to be compatible with the cipher communication, there is a need to remodel each application program as well as a hardware of a client or a server separately, so that the scale of remodelling as well as a process of remodelling are going to be considerable and in conjunction with these, a considerable sum of development costs are going to be required.
Now, the cipher communication described above is an effective security measure in the digital communication through a digital transmission path. In the cipher communication, the sender side and receiver side commonly share a predetermined identical cipher algorithm and a cipher key to be used in the cipher communication, and in general, the cipher communication is started and ended at some specific timings.
The cipher algorithm used in the conventional cipher communication includes FEAL (Fast data Encipherment ALgorithm), DES (Data Encryption Standard), etc., while a cipher key sharing scheme used in the conventional cipher communication includes the terminal-to-terminal key sharing scheme in which the key is exchanged between terminals, and the center key distribution scheme using a key distribution center. See, "Revised text of CD 9798-2, Entity authentication mechanisms, Part 2: Entity authentication using symmetric techniques, ISO/IEC/DP9798-2: 1990(E)" for further details.
As for a synchronization establishing scheme for the start and end of the cipher communication using stream data such as speech data through the digital transmission path, there has been a proposition of a scheme in which the sender side is set in the cipher communication state after transmitting a specific bit pattern indicating the start of the cipher communication at a timing for starting the cipher communication, while the receiver side is set in the cipher communication state when this specific bit pattern indicating the start of the cipher communication transmitted from the sender side is detected. See, Japanese Patent Application No. 2-192514 (1990).
On the other hand, as a scheme for establishing a synchronization of the cipher communication start/end for the data in the data communication, there is a scheme for regarding the data in the data communication as the stream data by using the physical layer as a unit of cipher processing just as in a case of the speech data, and the synchronization is established in the substantially similar manner as in a case of the speech data described above.
However, in this conventional synchronization establishing scheme for the start and end of the cipher communication of the data in the data communication, the encipher/decipher processing is applied directly to the digital data of the physical layer, so that when the bit error occurs on the transmission path, there are cases in which the synchronization processing or the error correction at the data link layer cannot be carried out.
For example, in a case of adopting the HDLC procedure as the protocol of the data link layer, the frame synchronization is realized by using the F pattern constituting the frame. Then, when the FEAL is used as the cipher algorithm, one bit error on the transmission path can be enlarged to 64 bits in the deciphered result depending on the cipher mode such that the F pattern itself contains an error, in which case the synchronization processing at the layer 2 is impossible.
Also, in a case of using the ISDN (Integrated Service Digital Network) line as the digital transmission line, if the encipher/decipher processing is applied to the physical layer data of the data in the data communication, this scheme becomes inapplicable when there is a need for the exchanger to interpret the content of the data channel as in the communication by the packet exchange. Consequently, the target of the cipher processing of the data in the data communication must be selected from the data other than that of the data link layer (see, K. Tanaka, I. Oyaizu, "A Confidentiality System for ISDN inter-PC High-Speed File Transfer", IEEE INFOCOM '94 proceedings, Vol. 3, pp. 1270-1277 (1994), and in such a case the synchronization establishing scheme for the cipher processing of the stream data such as speech data cannot be utilized.